Entropy And Spontaneity
For a certain reaction at 300 K, ΔH = −15 kJ mol⁻¹ and ΔS = −40 J K⁻¹ mol⁻¹. The Gibbs free energy change (ΔG) and the spontaneity of the reaction at this temperature are:
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About This Question
- Subject
- chemistry
- Chapter
- chemical thermodynamics
- Topic
- entropy and spontaneity
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
ΔG = −3 kJ mol⁻¹; spontaneous
- Formula: ΔG=ΔH−TΔS.
- Unit Consistency: Convert ΔS to kJ: ΔS=−40 J K−1=−0.040 kJ K−1.
- Substitute: ΔG=−15−(300)(−0.040)=−15+12=−3 kJ mol⁻¹.
- Spontaneity Check: ΔG < 0, so the reaction is spontaneous at 300 K.
- Conceptual Note: Although both ΔH and ΔS are negative (exothermic with decreased disorder), the reaction remains spontaneous at lower temperatures because the favourable enthalpy term dominates the unfavourable entropy term. At higher temperatures, TΔS could overcome ΔH and make ΔG positive.
This medium difficulty chemistry question is from the chapter chemical thermodynamics, covering the topic of entropy and spontaneity. It appeared in the 2025 exam.
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